Switch control circuit

ABSTRACT

A switching circuit controls at least one semiconductor switch TR 1  using drive signals generated from logic circuitry  4  that generates complementary drive signals on an upper and lower drive path. Validation circuitry is provided between the logic circuitry and the semiconductor switch that inhibits any uncontrolled switching of the semiconductor switch TR 1  due to noise, such as lightning strike, appearing at the load.

[0001] The present invention relates to a control circuit for a leastone semiconductor switch in which uncontrolled switching of thesemiconductor switch due to the presence of noise is inhibited.

[0002] There are many applications in which it is desirable to switch avoltage or load using a control signal. A common use of such a switchingarrangement is to switch a supply voltage on and off to an external orinternal load. An occurrence of this application is within the aerospaceindustry. Previously it was well known to accomplish such switchingusing an electromechanical relay. However, the electromechanical relayhas been replaced in many applications by semiconductor switches toachieve the higher reliability, lower cost, and ease of manufacture thatsemiconductor switches offer.

[0003] The use of semiconductor switches as replacements for amechanical relay can however also pose problems when a semiconductorswitch not directly connected to a common ground or supply rail isrequired. When using such a floating “high side” semiconductor switch itis desirable to limit as far as possible spurious or uncontrolledswitching of the semiconductor switch due to the presence of noise.Although such noise isolation may be achieved using wound or opticallyisolated semiconductor switches, such devices are ordinarilyprohibitively expensive and complex for most applications. A knownapproach has therefore been developed that makes it possible to drive afloating “high side” power semiconductor switch whilst avoidingtransformers, opto-isolators or individual DC to DC derived supplies.This known approach involves deriving a control voltage that is appliedto the semiconductor switch using a charge pump. As it is necessary thatthe switching circuit does not rely on the output load or supply for itsoperation, it is necessary to use a differential drive method to derivethe control voltage. Using this arrangement, it is known to provide acapacitive separation barrier to provide some noise isolation and DCblocking between the differential drive circuit and the overall powersupply. The use of such a capacitive separation barrier allows thesemiconductor switch and its control circuitry to float with respect toDC or low frequency AC voltage, whilst the power and control signals aretransferred in a higher frequency domain.

[0004] The problem with capacitive separation alone is if largeamplitude AC signals are present on the supply line or load they cancause spurious turn on of the semiconductor switch, or moreproblematically, can prevent the semiconductor switch from turning off.This is a problem on DC systems but is even more acute with high voltageAC supplies, such as 115V 400 Hz AC supplies commonly found in aerospaceapplications.

[0005] Further problems associated with floating semiconductor switchesare the inductively induced switching transients that can causebreakdown of the semiconductor switch. On DC systems it is possible todeal with load transients by means of freewheeling diodes, but this isgenerally not an optimal solution. Furthermore, freewheeling diodes donot control line-based transients produced when the switched supplyeither has a long run of cabling or is derived from a transformerrectifier unit (TRU). On AC systems, freewheeling diodes are notapplicable and either active or passive snubbing is required to avoiduncontrolled avalanche conditions in the power switch elements.

[0006] The alternative is to accommodate such line and load inducedtransients using the semiconductor switch itself to actively limit thetransient voltages and capacitive input current surges or absorbinductive energy. Such a protection scheme has the advantage of highspeed inductive load switching, thus providing a closer approximation ofrelay type operation. However, the problem with this form ofself-protection is that it may not provide adequate protection againstlightning strikes. The ability of a semiconductor switch to absorb theenergy contained in a lightning strike is limited by, first the physicalsize of the semiconductor switch and second the required clampingvoltage. These limitations means that, in certain safety criticalapplications, the semiconductor switches would have to be vastlyoverrated to provide the necessary self-protection, leading to increasedcosts and physical size of the circuits.

[0007] According to the present invention there is provided a controlcircuit for at least one first semiconductor device, the circuitcomprising: a rectifier circuit having first and second inputs andarranged to generate a control signal for the at least one firstsemiconductor device when the first and second inputs are driven inanti-phase; and a validation circuit coupled to said rectifier circuitand the or each first semiconductor device, said validation circuitbeing arranged to inhibit uncontrolled switching of the or each firstsemiconductor device in the presence of noise.

[0008] The provision of a validation circuit interposed between therectifier circuit and the semiconductor switch inhibits uncontrolledswitching of the or each semiconductor device, which preferably acts asa switch. A further advantage is that the use of additional devices isavoided to provide protection against lightning strikes.

[0009] Preferably the validation circuit comprises a secondsemiconductor device arranged to allow charge flow to a charge storeonly when an alternating voltage difference between the first and secondinputs of the rectifier circuit exceeds a first threshold. It istherefore possible to provide a control circuit that exhibits goodcommon mode noise rejection because only differential voltages arepermitted to be conducted by the validation circuit.

[0010] Preferably, the charge store is connected to a control terminalof the or each first semiconductor switch. The charge store provides atime delay in switching off or on the or each first semiconductor switchthat tends to counteract any uncontrolled operation of the semiconductorswitch.

[0011] Preferably, the or each first semiconductor switch is a fieldeffect transistor. FET's provide a cost effective solution whilstexhibiting good switching characteristics, and low “ON” resistance.

[0012] Preferably, the first and second inputs of the rectifier circuitare high pass filtered. This further improves the noise rejectioncapabilities of the circuit.

[0013] Preferably, the validation circuit is arranged such that anegative voltage transient, for example due to a lightning strike,applied to the or each first semiconductor switch biases said secondsemiconductor device off. The validation circuit is therefore arrangedto actively bias the second semiconductor device off in the event of anegative voltage strike, thus preventing uncontrolled turn on of thefield effect transistor. Additionally, the validation circuit mayfurther comprise a third semiconductor device arranged such that apositive voltage transient, for example due to a positive lightningstrike, applied to the or each first semiconductor device biases thethird semiconductor device off. The validation circuit thereforeprovides controlled protection of the field effect transistor in theevent of both negative and positive transients which may arise as aresult of lightning strikes.

[0014] Preferably, the control circuit further includes an inductiveload protection circuit for modifying the rate of turn off of the oreach first semiconductor switch so as to limit the voltage appearingacross the switch when switching loads having a significant imaginarycomponent as part of the load impedance.

[0015] Additionally, the protection circuit may comprise aunidirectional current flow path connection between a control terminalof the or each first semiconductor switch and a node formed between acapacitor and a resistor, a capacitor being in connection with a firstterminal of the semiconductor switch and the resistor being inconnection with a second terminal of the semiconductor switch.

[0016] It is therefore possible to additionally provide both absolutevoltage clamping and rate of change of voltage clamping when switchinginductive loads without the use of freewheeling diodes.

[0017] Embodiments of the present invention are described herein below,by way of example only, with reference to the accompanying figures, inwhich:

[0018]FIG. 1 schematically illustrates a DC control circuit according tothe present invention that provides common mode noise rejection;

[0019]FIG. 2 schematically illustrates a further example of an ACcontrol circuit according to the present invention that provides commonmode noise rejection;

[0020]FIG. 3 schematically illustrates a control circuit according tothe present invention providing noise protection against negativelightning strikes;

[0021]FIG. 4 schematically illustrates a control circuit according tothe present invention providing noise protection against both positiveand negative lightning strikes; and

[0022]FIG. 5 illustrates a voltage clamping circuit in accordance withembodiments of the present invention.

[0023]FIG. 1 illustrates a control circuit according to an embodiment ofthe present invention. In this embodiment, first semiconductor switchTR1 is used to switch a DC voltage provided by the DC power supply 2 tothe load L. A logic circuit 4 is arranged to provide complementaryoscillating drive signals on upper and lower drive paths 5 and 6,whenever it is desired to turn TR1 on. The anti phase signals from thelogic circuit 4 pass through serially connected capacitors and resistorsC1/R1 and C2/R2 and are rectified at a rectifier, generally indicated 7,using diodes D2 and D1 to produce, under no load conditions, a 10V-2Diode Drop square wave signal at the cathode of D1 relative to D2. Avalidation circuit 8 comprising further transistor TR2 is arranged toconduct the pulsed drive signal to the control terminal of thesemiconductor switch TR1 only when the polarity and magnitude of thevoltage at the cathode of D1 with respect to D2 is correct. In thisparticular embodiment TR1 is a field effect transistor. The secondtransistor is a PNP bipolar transistor having its emitter connected tothe cathode of D1 and its collector connected to the gate of TR1. Thebase of transistor TR2 is connected to the output of a voltage dividerformed by resistors R6 and R3 connected in series between the anode ofD1 and a cathode of a further diode D3 whose anode is connected to anode formed by TR1 and the load. The resistors R6 and R3 are also inparallel with D2. Due to this arrangement, only voltages that present avoltage across the diode D2 connected between the upper and lower drivepaths are rectified and therefore only such differential voltages aretranslated into a switching action by R6 and R3, allowing transistor TR1to conduct.

[0024] The control signal output from the collector of TR2 to the gate(ie a control terminal) of the first transistor TR1 is stored in acharge storage capacitor C3 which is in parallel with a dischargeresistor R4 so that the charge store discharges in a controlled way. Azener diode is provided in parallel with the charge store to provideprotection against over voltage conditions. The capacitor C3 is shown asa discrete component. However, in some embodiments of the presentinvention the storage capacitor can be formed by parasitic capacitancesuch as that associated with the gate of a field effect transistor.

[0025] The advantage exhibited by the embodiment shown in FIG. 1 is thatbecause only differential voltages applied across diode D2 causeswitching of field effect transistor TR1, the circuit rejects any commonmode noise.

[0026] A further embodiment of this circuit is shown in FIG. 2. In thisparticular embodiment the main power supply 2 is an AC supply and it istherefore necessary to provide a second field effect transistor, TR3, inthe switching path of the load L. It is necessary to provide two fieldeffect transistors for switching the alternating supply because theparasitic diodes 10, 12 of transistors TR1 and TR3 respectively, formedbecause of the diffusion between the drain and source of the device,mean that the MOSFET structure is not symmetrical. In the example shownin FIG. 2, the basic circuit of the differential charge pump, i.e. R1/C1and R2/C2 is the same as that illustrated in FIG. 1 and this functionsin the same way. However, the logic circuit 4 is shown as comprising asimple oscillator, configured around a dual op-amp to provide thedifferential clock, the clock being gated by a switch. It will beappreciated that other alternatives exist for the logic circuit 4 andthat the specific arrangement of the logic circuit 4 does not form apart of the present invention.

[0027]FIG. 3 illustrates a further embodiment providing increased noiseimmunity. As in the previous embodiments, logic circuitry 4 providescomplementary drive signals to capacitors C1 and C2. A voltage dividercomprising resistors R3 and R6 is also provided between the normal andcomplementary drive signals. However, in the circuit shown in FIG. 3,the semiconductor switch, TR2 is now placed in the lower drive path, asopposed to the upper drive path in FIGS. 1 and 2. Transistor TR2 is nowan NPN transistor as opposed to a PNP transistor shown in FIGS. 1 and 2.The remainder of this circuit is analogous to that shown in FIGS. 1 and2. As before, common mode noise rejection is provided because onlydifferential voltages applied across D1 cause TR2 to be conductive andtherefore to allow switching to occur at transistors TR1 and TR3.However, in addition the circuit shown in FIG. 3 provides improved noiseimmunity. Consider the case that a negative voltage transient caused bya lightning strike occurs on either drain of the output field effecttransistors. The negative spike would pass through the parasitic diodes10, 12 of the transistors and be applied to the collector of the firstNPN transistor TR2. As will be appreciated by those skilled in the art,the NPN transistor TR2 can be considered to be formed of two diodes, afirst diode being connected between the base and collector and a seconddiode being connected between the base and the emitter, with both anodesbeing connected to the base. Hence the negative voltage spike will passfreely in the reverse direction through the base/collector diode but isblocked by the base/emitter diode. The negative voltage then passesthrough resistor R3 and in the reverse direction through diode D1. Thevoltage dropped across diode D1 means that the upper drive path is morenegatively charged by the noise spike than the lower drive path. Thismeans that the base-emitter of transistor TR2 is reverse biased and istherefore actively held off, thus preventing the field effecttransistors TR1 and TR3 from being switched on.

[0028] Further circuit protection is provided because the negative spikemust flow through at least one of the voltage divider resistors R3 andR6, thus limiting the magnitude of current flow.

[0029] Should the field effect transistors TR1 and TR3 have beenswitched on at the point of the lightning strike occurring, the negativevoltage appearing at the gates of the transistors TR1 and TR3 will causethem to turn off and for the behaviour of the circuit to be the same asjust previously described. However, the output of the field effecttransistors will remain on for at least 200 μs due to the time constantcreated by the 20 Kohm resistor 14 and 10 nF capacitor C4. A lighteningstrike is usually less than 200 μs therefore its effect would not beseen. If the pulse were longer than 200 μs, then the FETS would turnoff.

[0030] Should a positive transient as a result of a lightning strikeoccur, the diodes D4 and D6 in the upper drive path block the path ofthe positive voltage. As the base/emitter voltage of transistor TR1 isfixed, then the positive pulse will simply raise the collector voltagetowards its maximum acceptable limit, because the transistor is behavingas a constant current device at this particular point in time. Thepositive voltage is prevented from travelling past the diodes in theupper drive path, and hence the switching voltage applied to transistorTR1 is unaffected and the circuit therefore behaves as normal.

[0031]FIG. 4 illustrates a further embodiment of the present inventionthat provides improved noise immunity against both negative and positivevoltage strikes. The circuit is analogous to that shown in FIG. 3 withthe addition of a further PNP transistor TR4 in the upper drive path.Additional transistor TR4 is switched using a voltage divider comprisingresistors R13 and R16 in an analogous manner to NPN transistor TR2. ThePNP transistor TR4 can also be considered to comprise of two diodes withtheir cathodes both connected to the base of the PNP transistor TR4 andtheir anodes being connected to the emitter and collector respectively.Should a positive lightning strike occur at either of the field effecttransistors TR1 and TR3 when those transistors are off, the diode formedbetween the collector and base of the PNP transistor TR4 will be forwardbiased and conduct the positive voltage. However, the NPN transistor TR2and the other diodes in the circuit prevent the positive voltage frompropagating any further to the left of the circuit. The net effect isthat the base of the PNP transistor TR4 is driven positive by thelightning strike and is thus therefore maintained in the off condition,thus maintaining the field effect transistors TR1 and TR3 in the offcondition. The PNP transistor TR4 will completely block a negativelightning strike and prevent negative voltage from passing any furtherto the left of the circuit. A negative lightning strike would maintainthe field effect transistors TR1 and TR3 in their off state.

[0032] Should a positive lightning strike appear at either the gate orthe source of the field effect transistors TR1 and TR3 whilst they arein their on state, then the PNP transistor TR4 will be turned offbecause, as explained above, the positive strike will propagate throughto the base of the PNP transistor, therefore turning it off. As the NPNtransistor TR2 has a fixed base/emitter voltage, the positive pulse willsimply raise the collector voltage of transistor TR2, as this transistoris effectively a constant current device at this time. The circuitfunctionality will therefore remain unchanged and the threat to thelogic drive circuit 4 will be within acceptable limits.

[0033] Should a negative lightning strike appear at either the gate orthe source of the field effect transistors TR1 and TR3 whilst they areon, then the collector/emitter voltage increases. As the PNP transistorTR4 is also acting as a constant current source, the gate/source voltageof the field effect transistors TR1 and TR3 remain unchanged because thecurrent through resistor R8 connected between the gates of transistorTR1 and TR3 remains the same. Thus the circuit operation is unaffected.

[0034]FIG. 5 schematically illustrates an embodiment of the presentinvention in which additional circuitry is provided to the switchingcircuit of the previously described embodiment, the additional circuitrybeing provided to allow large reactive loads to be switched by thecircuit and hence can be regarded as an inductive load protectioncircuit 20. Only the field effect transistor TR1, resistor R4 and diodeD4 of the switching circuit is shown in FIG. 5. The remainder of thecircuitry is as shown in the previous diagrams. Diodes D5 and D6 areconnected in series and with reverse polarity to each other between thegate of the field effect transistor TR1 and a supply voltage. Diodes D5and D6 provide absolute voltage clamping that prevents the voltage atthe source of the field effect transistor from exceeding a preset level.The rate of change of voltage with time across transistor TR1, referredto as Dv/Dt, is also controlled using the additional components R7, D7and C4. D7 and C4 are connected in series between the transistor gateand drain, with resistor R7 being connected between the lower path andthe junction of the capacitor C4 and diode D7. A suitable selection ofvalues for the circuit components enables the turn off of large reactiveloads whilst maintaining the previously described lightning strikeprotection. A small amount of let through, that is supply voltageappearing at the output, is permitted with this circuit arrangement. Inthis embodiment C3 has been omitted and the parasitic capacitance of thecircuit functions as the storage capacitor.

[0035] It is thus possible using embodiments of the present invention toprovide a switching circuit for a semiconductor switch that has improvednoise immunity and that can be used for switching reactive loads.

1. A control circuit for at least one first semiconductor device, thecircuit comprising: a rectifier circuit having first and second inputsand arranged to generate a control signal for the at least one firstsemiconductor device when the first and second inputs are driven in antiphase; and a validation circuit coupled to said rectifier circuit andthe or each first semiconductor device, and in which said validationcircuit is arranged to inhibit uncontrolled switching of the or eachfirst semiconductor device in the presence of noise.
 2. A controlcircuit as claimed in claim 1, wherein the validation circuit comprisesa second semiconductor device arranged to allow charge flow to a chargestore only when an alternating voltage difference between the first andsecond inputs of said rectifier circuit exceeds a threshold.
 3. Acontrol circuit as claimed in claim 2, wherein the charge store isconnected to a control terminal of the or each first semiconductordevice.
 4. A control circuit as claimed in claim 1, wherein the or eachfirst semiconductor device is a charge controlled device.
 5. A controlcircuit as claimed in claim 1, wherein the first and second inputs ofsaid rectifier circuit are high pass filtered.
 6. A control circuit asclaimed in claim 2, wherein said validation circuit is arranged suchthat a negative voltage transient applied to the or each firstsemiconductor device biases said second semiconductor device off.
 7. Acontrol circuit as claimed in claim 2, wherein said validation circuitfurther comprises a third semiconductor device arranged such that apositive voltage transient applied to the or each first semiconductordevice biases said third semiconductor device off.
 8. A control circuitas claimed in claim 1, further including an inductive load protectioncircuit for modifying the rate of turn off of the or each firstsemiconductor device so as to limit the voltage appearing across thedevice when switching loads having a significant imaginary component aspart of the local impedance.
 9. A control circuit as claimed in claim 8,in which the protection circuit comprises a uni-directional current flowpath connection between a control circuit of the first semiconductordevice and a node formed between a capacitor and a resistor, thecapacitor being in connection with a first terminal of the firstsemiconductor device and the resistor being in connection with a secondterminal of the first semiconductor device.
 10. A control circuit asclaimed in claim 1, wherein said validation circuit comprises at leastone impedance arranged to limit reverse current flow at said rectifiercircuit inputs during transient voltage conditions.
 11. A controlcircuit as claimed in claim 2, wherein the first semiconductor device isa field effect transistor and the charge store is formed by parasiticcapacitance associated with a gate of the field effect transistor.